1. Field of the Invention
The present invention relates to image data processing apparatus and method for use in image compression processing or the like.
2. Description of the Related Art
The high-efficiency image coding technique, on condition that images are related to communication media and recording media, extensively exploits DCT (Discrete Cosine Transform) approaches. However, the compression method using DCT has an essential problem that the image compression rate is limited because block distortion, mosquito noise and so on are visually recognized when the rate is increased.
Thus, in recent years, innovative image compression methods have been proposed in order to raise the limitation of compression rate. Particularly, a compression technique using wavelet transform as a kind of sub-band encoding has been attracting a great deal of attention. Use of this wavelet transform which has no idea of blocks will visually improve picture quality to a great extent since there is no such interblock distortion as in DCT.
As compared with compressed image in wavelet transform, the compressed image in DCT has a digital tendency, and contains much high frequency components under high compression rate. As a result, the high frequency components are preserved, but contrarily constitute conspicuous distortion. On the other hand, the wavelet transform gives rise to compressed image of analog tendency, and the high frequency components are naturally decreased with the increase of compression rate. In other words, the compressed image gradually loses signal components from the high frequency components of the signal band, thus resolution being totally reduced. If the compression rate in wavelet transform is the same as in DCT, visual picture quality deterioration is less than in DCT.
A conventional wavelet image compression apparatus will be described with reference to FIG. 12. When moving image data are compressed, input image data are supplied to a frame memory 2101, and the output data of the frame memory 2101 is fed to a wavelet transform unit 2102.
Here, the wavelet transform will be mentioned. FIG. 13 is a block diagram showing the wavelet transform processing in the wavelet transform unit 2102. As illustrated in FIG. 13, the input image data is fed to horizontal low-pass filter (LPF) and high-pass filter (HPF), so that the horizontal frequency band is divided into two parts. Then, the amount of data of each part is thinned out by a down sampler (indicated by down arrow) to an extent of 1/2 with respect to time. The 1/2-thinned image data produced from the down sampler through the horizontal low-pass filter (LPF) is further supplied to vertical low-pass filter (LPF) and high-pass filter (HPF), so that the vertical frequency band is divided into two parts. Then, the amount of data of each part is thinned out by a down sampler (indicated by down arrow) to an extent of 1/2 with respect to time.
Layer-0 sub-band signals will be described. The image data, which are thinned out to 1/2 through the horizontal low-pass filter (LPF) and further thinned out to 1/2 through the next vertical high-pass filter (HPF), is taken as an L0, LH sub-band signal. The image data, which are thinned out to 1/2 through the horizontal high-pass filter (HPF) and further thinned out to 1/2 through the next vertical low-pass filter (LPF), is taken as an L0, HL sub-band signal. The image data, which are thinned out to 1/2 through the horizontal high-pass filter (HPF) and further thinned out 1/2 through the next vertical high-pass filter (HPF), is taken as an L0, HH sub-band signal.
The component, which is thinned out to 1/2 through the first-stage horizontal low-pass filter (LPF) and further thinned out to 1/2 through the next vertical low-pass filter (LPF), is again subjected to the same processing as above (layer-1 and layer-2). In other words, L1, LH; L1, HL and L1, HH indicate layer-1 sub-band signals, and L2, LH; L2, HL and L2, HH indicate layer-2 sub-band signals. FIG. 13 shows the situation in which the input image data are three times subjected to wavelet transform for convenience of explanation.
The repetition of such processing gives rise to coefficient components. The horizontal and vertical frequency bands are divided, and coefficient data, in which the amount of data is reduced to 1/2 along the low-frequency region, are accumulated. FIG. 14 is a diagram to which reference is made in explaining a wavelet space, or showing wavelet transform coefficient signals for a plurality of frequency bands. In FIG. 14, reference numeral 1501 denotes a layer-2 LL-component sub-band space, 1502 a layer-2 HL-component sub-band space, 1503 a layer-2 LH-component sub-band space, and 1504 a layer-2 HH-component sub-band space. Reference numerals 1505 to 1507 denote layer-1 HL-, LH- and HH-component sub-band spaces, respectively. Reference numerals 1508 to 1510 denote layer-0 HL-, LH- and HH-component sub-band spaces, respectively.
Referring back to FIG. 12, the coefficient for each frequency band, which is produced by the wavelet transform unit 2102 having the above feature, is quantized by a quantizing unit 2103. The output data of the quantizing unit 2103 are coded by a variable-length coding (VLC) unit 2104, so that more information is assigned to data which occur at higher probability. In this way, the amount of information about whole data is reduced, and the input image is compressed.
However, although there are a lot of zero-value areas in a high-frequency component image space in the wavelet transform, the conventional wavelet transform apparatus cannot effectively reduce the amount of data in the zero-value regions. The conventional apparatus simply reduces the amount of data by making the quantizing step rough in the whole areas of the space which includes much zero value. Since picture quality is deteriorated if compression rate is increased, high compression rate cannot be achieved.